This invention relates to ion-plating apparatus for coating a substrate with a desired material.
A vacuum evaporating device comprises a source of a material to be evaporated and a holder for a substrate on which the material is to be evaporated. In contrast, conventional ion-plating apparatus comprises a source of the material to be ion-plated and means for applying a d.c. electric voltage between the source and substrate holder. In operation, the d.c. voltage produces a glow discharge in an atmosphere of an inert gas, such as argon or neon, of between 10.sup.-.sup.1 and 10.sup.-.sup.2 Torr. The glow discharge causes the material of the source to be sputtered, ionizes the particles of the material sputtered from the source, and accelerates the ionized particles towards the substrate to impinge thereon and adhere thereto.
Inasmuch as the d.c. voltage contributes to ionization and acceleration, the film obtained by ion-plating apparatus more tenaciously adheres to the substrate than a similar film coated by a vacuum evaporation device. In addition, it is possible with ion-plating apparatus to select the materials of the source and substrate from a wide variety of materials in view of the principles of operation thereof.
It has, however, been difficult with conventional ion-plating apparatus to control the temperature of the substrate because the substrate is bombarded not only by the ionized particles of the sputtered material but also by ions of the inert gas. This, in turn, renders it practically impossible to coat a substrate incapable of enduring a high temperature. In addition, it has often been experienced that back sputtering occurs during ion-plating to damage the coating growing on the substrate. It has now been found that these defects of conventional ion-plating apparatus result from the relatively high pressure at which ion-plating is carried out. Furthermore, the structure and characteristics of the coating obtained by conventional ion-plating apparatus have not been satisfactory for some applications.